The present disclosure is directed to a low sulfur nickel base substrate alloy and overlay coating system which results in excellent high temperature oxidation resistance.
For high temperature application of such components as blades and vanes in aeroengines or industrial gas turbines, equiaxed, columnar grain, or single crystal nickel base superalloys are typically used to achieve structural strength. Single crystal implies that the atomic arrangement all through the material is highly ordered and specific crystallographic directions can be associated with the material from one end to the other. When a material is made up of a large number of single crystal grains randomly oriented, the structure is considered to be equiaxed. If there are multiple columns of single crystal grains bundled together, the structure is referred to as columnar grain. Since such structure can only be generated using directional solidification, it is often referred to as directionally solidified material.
Superalloys refer to a class of nickel base alloys typically with alloying additions of Cr, Co, Mo, W, Re, Ru, Ti, Ta, and Al. Of these, the primary purpose of adding Al is to produce a dispersion of precipitates referred to as γ′, which is known to be an ordered intermetalic with structure similar to Ni3Al. The dispersion of γ′ precipitates, which is about 0.3 to 0.5 microns in size, is contained in a matrix which is referred to as the γ matrix. Elements such as Ti and Ta are known to largely replace Al in the γ′ precipitates, while elements such as Mo and Re are known to exclusively stay in the γ matrix. Other elements partition between the precipitate and matrix in a complex manner. If these alloys have an equiaxed or columnar grain microstructure, then they typically contain minor alloying additions of C, B, Zr and Hf to enhance the strength of grain boundaries and consequently enhance ductility of the alloys. The presence of Al also serves another function in that it enables alloys to form a protective alumina layer, which slows down the loss of metal due to oxidation in high temperature environment.